Modelling of soil-structure-interaction for flexible caissons for offshore wind turbines

Skau, Kristoffer Skjolden; Jostad, Hans Petter; Eiksund, Gudmund; Sturm, Hendrik

doi:10.1016/j.oceaneng.2018.10.035

Cited by 31 publications

(12 citation statements)

References 25 publications

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“…The matrix L, which accounts for the stiffness at a load reversal point, is defined as: where e is the elastic stiffness matrix and k vv , k hh , k mm and k hm define the vertical, horizontal, rotational and coupled horizontalrotational stiffness of the foundation system respectively. The influences of stiffness for the lid and the skirt have been comprehensively discussed in the work of Skau et al [49]. Note that the caisson foundation in this study is regarded as a rigid body.…”

Section: Development Of Macroelement Modelmentioning

confidence: 99%

“…Note that the caisson foundation in this study is regarded as a rigid body. The flexibility of the lid and the skirt could be considered as suggested by Skau et al [49]. As was the case for pile foundations [46,47], the coupled effect between horizontal forces and moment must be considered for the caisson foundation because of the skirt.…”

Section: Development Of Macroelement Modelmentioning

confidence: 99%

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A hypoplastic macroelement model for a caisson foundation in sand under monotonic and cyclic loadings

et al. 2019

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Nowadays, the numerical analysis of caisson foundations of offshore structures is a big challenge in engineering design. A simple, fast and accurate numerical tool is proposed in this article based on the macroelement concept. The novel macroelement is within the framework of hypoplasticity and can consider static monotonic and cyclic combined (multidirectional) loads for a caisson foundation in sand. The incremental nonlinear constitutive formulas are defined in terms of generalised forces and displacements and an enhanced function of failure surface is introduced. A series of well-documented laboratorial reduced-scale 1g model tests are adopted to validate the novel numerical tool. Results demonstrate a satisfied predictive performance of the proposed macroelement that can be used in caisson foundation design and constitutes an alternative to the traditional finite element analysis.

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Section: Development Of Macroelement Modelmentioning

confidence: 99%

Section: Development Of Macroelement Modelmentioning

confidence: 99%

A hypoplastic macroelement model for a caisson foundation in sand under monotonic and cyclic loadings

et al. 2019

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“…Appendix -3: Computation of vertical stiffness for shallow foundations k v Table A.1 provides guidance on how to compute for shallow embedded foundations. It must be mentioned that the method presented in this paper assumes a "linear" response of the foundations to obtain the natural frequencies For rigid shallow caissons in homogenous, parabolic, and linear ground profiles Solution for vertical stiffness of caissons provided in tabular format and is dependent on relative soil to pile stiffness, embedment ratio, and ground profile stiffness variation with depth (Skau, et al, 2018) [26] For flexible shallow suction caissons. Dependent on finite element soil model for the extraction of the macro-element model Adjusted the macro-element model provided in [27] (which assumes rigid behaviour) where the bending of the caisson lid in the vertical direction inherently reduces the stiffness of the foundation in addition to changing the volume of the soil plug, i.e.…”

Section: Square Foundation Arrangementmentioning

confidence: 99%

Dynamic design considerations for offshore wind turbine jackets supported on multiple foundations

et al. 2019

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To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT's) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates at least two limit states: Fatigue Limit State (FLS) and overall deformation in the Serviceability Limit State (SLS). It has been observed through scaled model tests that the first eigen frequency of vibration for OWTs supported on multiple shallow foundations (such as jackets on 3 or 4 suction caissons) corresponds to low frequency rocking modes of vibration. In the absence of adequate damping, if the forcing frequency of the rotor (so called 1P) is in close proximity to the natural frequency of the system, resonance may occur affecting the fatigue design life. A similar phenomenon commonly known as "ground resonance" is widely observed in helicopters (without dampers) where the rotor frequency can be very close to the overall frequency causing the helicopter to a possible collapse. This paper suggests that designers need to optimise the configuration of the jacket and choose the vertical stiffness of the foundation such that rocking modes of vibration are prevented. It is advisable to steer the jacket solution towards sway-bending mode as the first mode of vibration. Analytical solutions are developed to predict the eigen frequencies of jacket supported offshore wind turbines and validated using the finite element method. Effectively, two parameters govern the rocking frequency of a jacket: (a) ratio of super structure stiffness (essentially lateral stiffness of the tower and the jacket) to vertical stiffness of the foundation; (b) aspect ratio (ratio of base dimension to the tower dimension) of the jacket. A practical example considering a jacket supporting a 5MW turbine is considered to demonstrate the calculation procedure which can allow a designer to choose a foundation. It is anticipated that the results will have an impact in choosing foundations for jackets.

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“…Following the approach by Liingaard et al, 24 more recently, a significant number of papers were published investigating the kinematic interaction of caissons foundations exploiting a finite element modelling approach, taking advantage of the continuous IT advances leading to ever more performing computers. [25][26][27][28][29] However, despite high performant computers are nowadays available, finite element or boundary element modelling strategies still remain computationally demanding procedures for the analysis of SSI problems because a significant portion of soil (usually not only the near-field portion) must be included in the models due to the non-perfect radiation condition at the model boundaries, which is usually simulated numerically through infinite elements or Lysmer-Kuhlemeyer dashpots. 30 Consequently, the above methods are not suitable for phenomenological or probabilistic investigations of the dynamic response of caisson foundations on a large scale, since they imply a parametric scheme to study the effects on impedances and on the FIM due to the variability of the geometric and mechanical parameters of the problem, which may be very high, especially if stratified soil conditions are taken into account.…”

Section: Introductionmentioning

confidence: 99%

Winkler model for predicting the dynamic response of caisson foundations

Carbonari

Bordón

Padrón

et al. 2022

Earthq Engng Struct Dyn

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The paper presents a Winkler‐based numerical model for the analysis of the dynamic response of caisson foundations. The model allows the evaluation of the impedance functions and of the foundation input motion (FIM), which can be used in the framework of the substructure approach to compute inertial soil‐foundation superstructure interaction analyses. In addition, kinematic stress resultants due to seismic shear waves propagating into the soil can be estimated. The caisson is modelled as a Timoshenko beam and the soil‐caisson interaction forces are derived from the analyses of the plane‐strain vibration problem of an annular rigid ring embedded into the soil. The problem solution is obtained in the frequency domain exploiting the finite element approach and generic soil stratigraphies can be considered in the applications. The model, which is characterised by a very low computational effort, is validated by performing a parametric investigation, comparing results with those obtained from more rigorous BEM‐FEM models of the soil‐caissons systems. Finally, some applications to real caisson foundations of offshore wind turbines (OWTs) are shown to demonstrate the model accuracy in capturing the seismic response of the foundations obtained from more rigorous models.

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Modelling of soil-structure-interaction for flexible caissons for offshore wind turbines

Cited by 31 publications

References 25 publications

A hypoplastic macroelement model for a caisson foundation in sand under monotonic and cyclic loadings

A hypoplastic macroelement model for a caisson foundation in sand under monotonic and cyclic loadings

Dynamic design considerations for offshore wind turbine jackets supported on multiple foundations

Winkler model for predicting the dynamic response of caisson foundations

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